It is far from optimal. It's just a last resort. Maybe you and a friend are out canyoneering one day, you both slip and fall, and a boulder falls and pins both of you against a wall. He or she has a crappy phone that cannot get a signal but which happens to have a lot of battery power left, and you have an iPhone 6s Plus with no battery power left. You could charge your battery from theirs and make the phone call that saves the day.
The thing is only $11, but it might save your life.